Recently, semiconductor devices have decreased in size, and this situation has required semiconductor manufacturing apparatuses as well as inspection and/or evaluation apparatuses to have higher precision correspondingly. Generally, in order to evaluate whether a shape and/or dimensions of a pattern formed on a semiconductor wafer are accurate or not, a scanning electron microscope having a length measurement function (hereinafter, called a length measurement SEM) is used.
The length measurement SEM irradiates, with an electron beam, a wafer on which a pattern is formed, manipulates a secondary electron signal obtained from the projection for image processing, and derives a dimension by finding out an edge of the pattern from signal's light-dark transition. Then, in order to correspond to a decrease in size of a semiconductor device described above, it is important to obtain a secondary electron image having a less amount of noise in a higher observation magnification. For the purpose, it is necessary to improve contrast by superposing many secondary electron images on one another, and a sample stage for mounting and holding a wafer is required to be quite precisely positioned in the micron order.
Further, the travel time of a sample stage has a large effect on the throughput of an entire device. Particularly, in the case of a sample stage which transports a large semiconductor wafer, the travel time of the sample stage is required to be reduced for improving the throughput.
On the one hand, a larger and/or heavier stage device increases the mass of a sample chamber which houses the stage device, and also increases costs of a mount for holding the sample chamber and setting of the mount. Therefore, the stage device has to be smaller and lighter.
As described above, the stage device is required to have capability of a high precision and high speed positioning, and simultaneously to be smaller and lighter. For the purpose, a servo control system is commonly employed which includes a drive mechanism such as a linear motor. However, in the case where the stage device with a linear motor is applied to a length measurement SEM, an electron beam is deflected by a magnetic field generated from a primary side formed of a coil and a secondary side formed of a permanent magnet of the linear motor, and a secondary electron image may accordingly be adversely affected. The length measurement SEM moves a stage to a measurement point on a semiconductor wafer, and subsequently acquires an image using an electron beam. For the purpose, the length measurement SEM can eliminate a magnetic field generated from a primary side by suspending power supplying to a coil after positioning the stage. However, because a secondary side uses a permanent magnet, a magnetic field cannot be completely eliminated. Accordingly, it is necessary to design in consideration of the effect on a secondary electron image to be acquired using the electron beam.
As described above, the magnet fields which may adversely affect the electron beam are roughly classified into two types. One of the two types is called a fixed magnetic field having the absolute amount of the magnetic field at an electron beam projection position. The other is a varying magnetic field in which a magnetic field at the electron beam projection position varies when the stage is moved. Since the fixed magnetic field does not vary independently of the movement of the stage, its effect can be eliminated to a certain extent by compensating for deflection of an electron beam, which compensation is performed, for example, based on a distribution map of the fixed magnetic field acquired in advance. However, if the absolute amount of the magnetic field is large, it may be thought that the compensation by the deflection of the electron beam cannot be sufficient. Accordingly, the fixed magnetic field has to be decreased in amplitude. Further, since the varying magnetic field varies when the stage is moved, the compensation cannot be executed in advance. Accordingly, it is necessary to control the variation in magnetic field when the stage is moved.
Generally, there is a system configured for providing a stage device which operates in the X-Y plane, the system including a linear motor and a linear guide disposed in the direction of one axis (here, called an upper axis) on a table which moves in the direction of the other axis (here, called a lower axis), and a top table disposed on a linear motor mover in the upper axis. According to this system, because permanent magnets of the linear motor in the upper axis moves together with the table in the lower axis, the variation in magnetic field at an electron beam projection position is very large.
As measures against the fixed magnetic field and the varying magnetic field which affect the electron beam as described above, the technology has been described in Patent Literature 1 which reduces the effect of the magnetic fields by distancing the magnetic fields from the electron beam projection position using four linear motors. In this technology, a movable table is configured to intersect with the X-direction and the Y-direction, and to operate in the X-Y plane by disposing linear motors on both ends of the movable table from the outside. Further, as another technology, the technology has been described in Patent Literature 2 which eliminates the effect of the magnetic fields on the electron beam by disposing linear motors outside of a sample chamber.